Subject matter of this invention relates to the application of suspended particulate matter to a newly formed web of paper in such as manner that the suspended material is captured or deposited within the fibers of the sheet such that a high percentage of the particulate matter, upwards of 50% or more, is retained within the web. The invention is particularly suited for the application of a sizing starch to a web, and provides an apparatus and method which can take the place of the conventional sizing press, although other kinds of matter may be applied, or combined with starch.
Sizing in the form of starch has commonly been applied to newly formed paper webs to enhance the mechanical properties of the paper. Particularly, starch has been found to have a significant impact on sheet strength properties, including tensile strength, stiffness, resistance against edgewise compression, and pick resistance. Starch can increase compressive strengths by about 25%. Commonly, starch solutions are applied by a size press, although other arrangements have been used.
Starch has also been applied, in various other manners, to a newly formed web at the so-called wet end of a paper machine. The prior art includes references which teach the direct application of a cooked starch solution to a newly-formed web on a wire of a fourdrinier machine. The early references of Olander et al., U.S. Pat. No. 1,538,582 of May 19, 1925 and Johnsen, U.S. Pat. No. 1,903,326 of Mar. 28, 1933 apply a sizing solution to an upper surface of a web by an overflow applicator. A more recent example of a starch application is the wet end curtain coater of Coleman, U.S. Pat. No. 3,992,252 issued Nov. 16, 1976.
Generally, the application of a starch solution by an overflow applicator at the wet end of a paper machine has not been widely practiced. The amount of starch which can be added is often insufficient to provide the desired properties to the finished paper. It also has been observed that cooked starch interferes with water drainage of the web on the wire.
The principal means of adding starch is by a conventional size press. Size presses are commonly used after a first dryer section, and have the capability of applying starch in typical pickup ranges from 40 lbs. per ton to 100 lbs. per ton or more. However, such installations suffer the disadvantages in the high cost of the size press and in the costs associated with the additional dryer sections and heat energy required downstream of the size press.
Another method of applying starch is to add the solution with the paper pulp stock prior to or at the headbox. Commonly, cationic cooked starch is used. About 35 lbs. of starch per ton of paper has been the practical upper limit for retention in the sheet. Since the starch is in solution, some of it drains out with the white water and, eventually, the white water loop will fill up with starch. This is one of the reasons why the amount of starch which can be added in a headbox has a practical limit. Also, where the starch is in solution, the paper fibers have a minimum filtering effect on the starch which would otherwise tend to retain the starch and for that reason, cationic attraction has been used to improve retention. However, ionic trash tends to neutralize the starch's cationic charge and reduces retention.
In other instances, particularly in the manufacture of multi-ply board on cylinder machines, spray bars have been used to apply a starch solution directly on the wet stock. Spray bar arrangements are not widely used at wet ends of paper machines due to the poor appearance of the paper by reason of a non-uniformity of the starch application.
Dry electrostically charged starch particles have been deposited on the surface of a web, while the web is on the wire of a fourdrinier machine, as described in the U.S. Pat. No. 3,919,042 of Spiller issued Nov. 11, 1975. Spiller teaches that such electrostatically charged particles of dry starch may be applied at relatively light weights (1-3% starch on the basis of fiber weight, i.e., 20-90 lbs. per ton). Also, Spiller teaches that the dry starch will, to some extent, be hydrated by moisture in the web and will be cooked as the web passes through the high temperature dryer section of the paper machine.
Attempts to add wet materials to an upper exposed surface of a newly formed and draining web of paper, such as on a wire of a fourdrinier machine, have suffered due to the lack of satisfactory application apparatus and methods. Starches have been applied variously by apparatus which allows a starch solution to fall along the surface a lip or wall, as a curtain and continue to the web. However, starch can build up on the applicator surfaces and result in uneven distribution and application of the material, or can form skips in the coating.
It is known that the application of heat to a film or curtain improves the ability of the suction boxes to remove the water content of the web, but inadequate attention has been paid to apparatus for permitting the application of heat to the coating material and at the same time preventing the coater parts lips from accumulating coating material that interferes with the uniform application of the coating.